Keep phoning my number and hang up when I answer

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I called back number (03) 5269 1542 reached IVR for MDM Systems.

Phone me but never leave message on answering system if I don't answer.

Blocked previously for shenanigans (can't recall what, but it was either not telling me who they wished to speak to when asked, deliberate scam call or just nuisance prank-calling), my home VoIP server now just hangs up on this number which causes my VoIP provider to redirect them to voice mail instead of ringing the phones here.

Today, they left a completely garbled and unintelligible message on my land-line voicemail (which is hosted on my VoIP provider's infrastructure, so not my Internet connection at fault).

Caller: no idea

keep phoning and hang-up when I answer.